The invention relates to the area of high pressure injectors, intended to inject water into a machine or installation containing a pressurized reservoir. Generally, the latter is the steam production tank of a steam boiler. This is the case in particular for steam generators used in nuclear reactors, especially pressurized water reactors. However, the use of this type of injector could be applied to any type of steam-producing reservoir using part of this steam as driving energy source and a low pressure reservoir as water source.
For over a century the use of steam injectors has been known (see GIFFARD patent in 1850), in particular for steam engines such as locomotives and ships. Nowadays, these devices are especially used in industrial installations requiring the decanting of solutions or liquid waste likely to rapidly deteriorate conventional pumping systems. In water nuclear reactors, the use of injectors as an emergency supply has been examined. Such supply is intended to evacuate residual heat. In pressurized water reactors, the emergency supply to steam generators is made using electric motor pumps or turbopumps. These devices are difficult to design on account of their revolving parts and some depend upon electric sources. On this account, the use of passive devices has been researched, such as steam injectors which are able to raise the pressure of the water in the low pressure emergency reservoir to a pressure greater than the steam pressure. Up until now, the different injector prototypes put forward have been found to perform insufficiently and to be unreliable for use in nuclear reactors.
With reference to FIG. 1, the principle of a steam injector is to reduce the pressure of pressurized steam within a narrowing followed by an expanding nozzle 2, a Laval nozzle for example, so that the speed reached on leaving this tube is a supersonic speed with pressures possibly lower than atmospheric pressure. In a mixing chamber 4, a water inlet is provided via a ring-shaped entry chamber 3. In the mixing chamber 4, the water derived from the entry chamber 3 is aspirated under the low pressures, then the steam releases its energy to the water by condensing.
The mixing chamber 4 is generally cone-shaped and converges towards a neck 5. At this point, the water reaches its maximum speed. After the neck 5 is an outlet diffuser 7 through which the kinetic energy of the diphase mixture is converted into pressure and is accompanied by condensation of the steam that is non-condensed on leaving the mixing chamber 4. This pressure rise is abrupt and is sometimes compared to a stationary shock wave. To ensure its start-up, the steam injector requires a drain 6 positioned at the mixing chamber 4. This start-up may also be difficult to achieve as the drain must be properly positioned. In addition, once the injector has been primed, closure of the drain 6 may cause de-energizing of the steam injector (in general gradual closure is recommended) The maximum outlet pressure is greater the smaller the section of the neck passageway 5 located between the mixing chamber 4 and the diffuser 7. However, reducing the size of this section renders start-up of the device even more difficult.
Moreover, the use of two drains 6 (FIG. 2) makes it possible for some injectors to reach pressures of 70 bars to 90 bars. In this case, only the upstream drain is closed during normal functioning of the steam injector, the downstream drain remaining more or less open to evacuate a fairly considerable quantity of water, approximately 50%, for high pressure operation. The complex functioning and loss of water from this type of steam injector have meant that it could not be chosen for nuclear reactor installations.
The purpose of the invention is therefore to overcome these disadvantages by making available a steam injector which may be used in pressurized water reactors and which may inject water up to pressures in the region of 80 bars.
Therefore, the main subject of the invention is a high pressure steam injector comprising:
a steam inlet leading into:
a steam nozzle itself leading into:
a mixing chamber;
a ring-shaped entry chamber leading into the mixing chamber;
a neck positioned at the mixing chamber exit,
a diffuser positioned at the neck exit; and
an outlet positioned downstream from the diffuser.
According to the invention, an axial drain formed of an evacuation duct is positioned in the middle of the neck to reduce the neck section and purge some of the steam which has not been condensed and to evacuate it towards the outside. It has been shown that flow remains essentially annular as far as the neck.
For the purpose of possibly using the drain temporarily or varying the minimum passageway section, the drain may be assembled with longitudinal mobility so that it can be moved relative to the neck.
To improve the efficacy of this drain, it may have a variable section.
A further embodiment provides a cone shape for the first part of the axial drain in which evacuation holes are provided, so that the steam can be drained progressively.